Image reading apparatus, and dimming control method and line sensor layout method therefor

ABSTRACT

In an image reading apparatus for reading an original image using a light source containing a plurality of color components, a control pulse for dimming the light source is generated by pulse-width modulation symmetrically with respect to a reference timing (for example, the central position in one storage time or the storage start timing) in one predetermined storage time of a line sensor.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an image reading apparatus forreading an image on an original, and dimming control method and linesensor layout method therefor.

[0002] Conventionally, various image reading apparatuses for forming theimage of image information on an original on a plurality of line sensors(solid-state image sensing elements such as CCDs) through an imagingoptical system and reading the image as monochromatic or color digitalimage information on the basis of output signals from the line sensorshave been proposed.

[0003]FIG. 29 is a schematic view showing principal part of the opticalsystem of a conventional color image reading apparatus.

[0004] Referring to FIG. 29, an original (not shown) placed on anoriginal glass table 100 is illuminated with a rod-shaped light source101. A reflecting shade 102 is used to improve the illuminationefficiency. Light from the original illuminated with the rod-shapedlight source 101 and reflecting shade 102 is guided to an imagingoptical system 104 through mirrors 103-a, 103-b, and 103-c. The imagingoptical system 104 forms the image of image information of the originalon a solid-state image sensing element (line sensor) 105.

[0005] The line sensor 105 comprises three line sensors independentlyprepared for R, G, and B signals. A light amount sensor 106 detects thelight amount of the rod-shaped light source 101. The rod-shaped lightsource is ON/OFF-controlled on the basis of the output from the lightamount sensor 106 such that the rod-shaped light source 101 emits lightin a predetermined amount.

[0006] As the mirror 103-a is moved by a mechanism (not shown) in amain-scanning direction B at a scanning speed v, and the mirrors 103-band 103-c move in a sub-scanning direction A at a speed of v/2 insynchronism with movement of the mirror 103-a. Thereby, the imagerepresenting an original surface of the main-scanning direction issequentially formed on the line sensor 101 as a solid-state imagesensing element. The image formed on the solid-state image sensingelement 105 is converted into an electrical signal, sent to an outputdevice (not shown), and printed, or sent to a storage device to storethe input image information.

[0007] As the light source of such an image reading apparatus, a halogenlamp is conventionally used. A halogen lamp has a high luminance.However, since this lamp exhibits a large increase in-temperature andrequires power consumption of 200 to 300 W, power consumption of theentire apparatus increases. In recent years, to avoid this problem,high-luminance fluorescent lamps or xenon lamps have been developed andused as light sources for image reading apparatuses.

[0008] Generally, a fluorescent lamp or xenon lamp seals a small contentof mercury mass and Ar or Kr, or Xe at several Torr in a rod-shapedhollow tube. Various phosphors are applied to the inner wall of thehollow tube, and electrodes are formed at the two ends of the hollowtube. In a fluorescent lamp or xenon lamp with this structure, UV raysare emitted from mercury or various gases upon discharge from theelectrodes, and accordingly, the phosphors applied to the inner wall ofthe tube are excited to emit visible light in accordance with thelight-emitting characteristics of the phosphors. Phosphors to beemployed are selected in accordance with spectral energy characteristicsrequired for a light source. Especially, a color image reading apparatusrequires a light source having a wide wavelength range corresponding toR, G, and B (red, green, and blue) components. When a light source witha particularly high luminance is necessary, phosphors of a plurality ofcolors are mixed and applied to the inner wall of a tube.

[0009] However, the above-described conventional image reading apparatushas the following disadvantages.

[0010] The light-emitting amount (light-emitting intensity) of afluorescent lamp or xenon lamp is generally controlled by pulse-widthmodulation (PWM) for controlling the pulse width corresponding to the ONtime while keeping the value of a current flowing to the lamp constant,unlike a halogen lamp which controls the lighting voltage. PWM isemployed because a fluorescent lamp or xenon lamp starts light emissionwhen the current value exceeds a predetermined value. If thelight-emitting amount is controlled by controlling the value of thecurrent to be supplied, the range of light-emitting amount controlbecomes narrow.

[0011]FIG. 30 shows a control waveform for controlling thelight-emitting amount of a fluorescent lamp by pulse-width modulation.In FIG. 30, the abscissa represents time, and the ordinate represents acurrent value for controlling the light-emitting amount of the lightsource. A period Hsync on the abscissa represents a time correspondingto a predetermined storage time of a solid-state image sensing element.This time corresponds to a time when charges are stored incorrespondence with the amount of light incident on the light-receivingportion of the solid-state image sensing element.

[0012] For normal pulse width control, a control signal is output onceper storage time in synchronism with the rise or fall of a triggersignal indicating the start of the period (period of time) Hsync as thestorage time. When dimming is controlled in synchronism with a signalcorresponding to the trigger signal of one storage time, noise due to abeat generated by interference between the storage time and pulse widthcontrol for controlling the light amount is removed from an imagesignal.

[0013] As a fluorescent lamp or xenon lamp coated with phosphors andused in an image reading apparatus for reading color image information,a white light source is often employed. In this light source, phosphorsof a plurality of colors are mixed and applied to the inner wall of thelamp to simultaneously emit light components of various colors, therebyobtaining light-emitting characteristics in a wide wavelength rangeacross the visible light range.

[0014] A white light source has a problem due to the difference inafterglow characteristics unique to the phosphors of different colors.Here, the afterglow characteristics mean that emitted light remains evenafter the current for controlling light emission of the light source isinstantaneously cut off. Afterglow characteristics depend on the timewhen a phosphor excited by UV rays is staying at a high energy level andgenerally decrease as an exponential function. Depending on thecharacteristics of the material of a phosphor, the afterglowcharacteristics can be represented by

T=e(τ−1)

[0015] where τ represents characteristics determined by the material ofa phosphor. When phosphors corresponding to R, G, and B colors aremixed, as in a white light source, τ changes in units of colors. Amaterial used as a phosphor is generally determined on the basis of thelight-emitting wavelength characteristics in a wavelength range,luminous efficiency, and service life of the material. Followingmaterials are often used.

[0016] Blue: BaMg₂Al₁₆O₂₇ (center wavelength 452 nm, T=2 μsec)

[0017] Red: Y₂O₃: Eu²⁺, (center wavelength 611 nm, T=1.1 msec)

[0018] Green: LaPO₄: Ce, Tb (center wavelength 544 nm, T=2.6 msec)

[0019] T is the attenuation time of each material when thelight-emitting amount reaches 1/e due to attenuation.

[0020] Since different colors have different afterglow characteristics(especially blue light has a short attenuation time), the barycenter ofa read position in the sub-scanning direction changes depending on thecolor. This phenomenon will be described with reference to FIG. 31.

[0021] The abscissa of the graph shown in FIG. 31 represents time, andthe ordinate represents the amount of a current for driving afluorescent lamp and the light-emitting amount of the fluorescent lamp.FIG. 31 shows the model of afterglow generated on the basis of theattenuation characteristics of the R, G, and B colors.

[0022] Normally, light amount control (also called dimming control ordimming) of a fluorescent lamp is performed once in the period Hsynccorresponding to one storage time of a solid-state image sensingelement. The solid-state image sensing element stores charges inproportion to the amount of incident light.

[0023] In FIG. 31, the dimming period corresponds to a time when acurrent for driving the fluorescent lamp is supplied in an amountproportional to the dimming duty. As a technique mainly used, thecurrent is switched to a high frequency during this period.

[0024] After the time corresponding to the dimming period, thelight-emitting amount decreases. The attenuation characteristics aredetermined by the following two factors. One is the attenuationcharacteristics of a bright line spectrum generated by the fluorescentlamp, and the other is the above-described attenuation characteristicsof the phosphor.

[0025] Normally, one storage time corresponding to the period Hsync isseveral hundred μsec. A bright line spectrum attenuates for 1 μsec orless and rarely influences. However, a phosphor attenuates on the orderof millisecond and considerably influences. Hence, the attenuationcharacteristics of a light-emitting amount are determined by the sum ofthe light-emitting amounts of two types and the attenuationcharacteristics of each light emission.

[0026] In a fluorescent lamp turned on by a substantially predeterminedcurrent to emit light in a substantially predetermined amount during thedimming period, the light amount corresponding to the bright linespectrum instantaneously decreases when the dimming period is ended.This corresponds to a portion L1. In addition, afterglow correspondingto a portion L2 is generated due to the attenuation characteristics ofthe fluorescent lamp.

[0027] The afterglow characteristics of color light components have thefollowing problem in an image reading apparatus.

[0028] One storage time of the solid-state image sensing element servesnot only as a reference time in reading image information but also as areference read position in reading in the sub-scanning direction.

[0029] The pixel density in reading image information is determined bythe pixel size of the solid-state image sensing element in the mainscanning direction, and the moving distance in image reading by mirrorscanning in the sub-scanning direction.

[0030] Hence, the phenomenon that the light-emitting amounts of colorlight components have different barycenter positions with respect to thetime Hsync because of their afterglow characteristics may be consideredby replacing the abscissa of the graph in FIG. 31 with positioninformation.

[0031] In FIG. 32, the abscissa is replaced with the distance in thesub-scanning direction. The barycenters (barycenter R and barycenter G)of the read positions of the R and G components in the sub-scanningdirection move with respect to the barycenter position (barycenter B) ofthe B component with the smallest afterglow amount by d2. When thebarycenter of the read position in the sub-scanning direction changes inunits of colors, color misregistration occurs in reading in thesub-scanning direction to degrade the performance of the image readingapparatus. In an image obtained by reading a thin black line, a phaseshift between the R, G, and B signals appears at the edge portion of thethin black line. If color misregistration occurs at the edge portion ofthe thin black line, the black thin line contained in the originalcannot be expressed, and the image quality is degraded.

[0032] For an image reading apparatus using a fluorescent lamp or xenonlamp, another technique is examined in which the above-described lightamount control is omitted, gain setting of an amplifier for electricallyamplifying an output signal from the solid-state image sensing elementis changed in accordance with a decrease in light amount due todurability, and an appropriate signal output is obtained by changing thegain in accordance with the decrease in light amount. However, when thegain is changed, the S/N ratio of the read signal varies depending onthe value of the gain.

SUMMARY OF THE INVENTION

[0033] The present invention has been made to solve the problems of theabove-described conventional image reading apparatus, and has as itsobject to provide an image reading apparatus which reduces the influenceof the afterglow characteristics of a light source in reading anoriginal image, and dimming control method and line sensor layout methodtherefor.

[0034] In order to achieve the above object, an image reading apparatusaccording to the present invention is characterized by the followingarrangements.

[0035] An image reading apparatus for irradiating an original image witha light source and forming an image corresponding to the original imageon an image sensing device through an imaging optical system to read theoriginal image is characterized by comprising control means for shiftingan ON start timing of the light source for illuminating the originalimage from a start timing of a predetermined charge storage period ofthe image sensing device.

[0036] An image reading apparatus for irradiating an original image witha light source and forming an image corresponding to the original imageon an image sensing device through an imaging optical system whilescanning the original image in main and sub-scanning directions to readthe original image is characterized by comprising control means forreducing barycenter movement of read positions of a plurality of colorcomponents in the sub-scanning direction, that is generated by afterglowcharacteristics of the plurality of color components contained in thelight source for illuminating the original image.

[0037] An image reading apparatus having a white light source containingR (red), G (green), and B (blue) color components with afterglowcharacteristics different from each other, and line sensors of R, G, andB colors, which are laid out with an offset in a sub-scanning direction,is characterized in that the relative layout of the line sensors of R,G, and B colors is determined on the basis of the afterglowcharacteristics of the R, G, and B color components of the white lightsource.

[0038] In order to achieve the above object, a dimming control methodfor an image reading apparatus according to the present invention ischaracterized by the following arrangements.

[0039] A dimming control method for a light source in an image readingapparatus for irradiating an original image with the light source andforming an image corresponding to the original image on an image sensingdevice through an imaging optical system to read the original image ischaracterized by comprising a control step of shifting an ON starttiming of the light source for illuminating the original image from astart timing of a predetermined charge storage period of the imagesensing device.

[0040] A dimming control method for a light source in an image readingapparatus for irradiating an original image with the light source andforming an image corresponding to the original image on an image sensingdevice through an imaging optical system while scanning the originalimage in main and sub-scanning directions to read the original image ischaracterized by comprising a control step of reducing barycentermovement of read positions of a plurality of color components in thesub-scanning direction, that is generated by afterglow characteristicsof the plurality of color components contained in the light source forilluminating the original image.

[0041] In order to achieve the above object, a line sensor layout methodfor an image reading apparatus according to the present invention ischaracterized by the following arrangements.

[0042] A line sensor layout method in an image reading apparatus havinga white light source containing R (red), G (green), and B (blue) colorcomponents with afterglow characteristics different from each other, andline sensors of R, G, and B colors, which are laid out with an offset ina sub-scanning direction, is characterized in that the relative layoutof the line sensors of R, G, and B colors is determined on the basis ofthe afterglow characteristics of the R, G, and B color components of thewhite light source.

[0043] Other features and advantages of the present invention will beapparent from the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044]FIG. 1 is a graph for explaining a dimming control method for animage reading apparatus according to the first embodiment of the presentinvention;

[0045]FIG. 2 is a graph for explaining the dimming control method forthe image reading apparatus according to the first embodiment of thepresent invention;

[0046]FIG. 3 is a schematic view showing the arrangement of dimming thatcan be applied to the image reading apparatus according to the firstembodiment;

[0047]FIG. 4 is a block diagram showing the schematic arrangement of thecontrol unit of the image reading apparatus according to the firstembodiment of the present invention;

[0048]FIG. 5 is a block diagram showing details of the arrangement ofthe control unit shown in FIG. 4;

[0049]FIG. 6 is a block diagram showing details of a delay adjustmentcircuit included in the control unit shown in FIG. 4;

[0050]FIG. 7 is a block diagram showing details of a down counterincluded in the control unit shown in FIG. 4;

[0051]FIGS. 8A and 8B are timing charts for explaining a dimming controlmethod according to the first embodiment of the present invention;

[0052]FIGS. 9A and 9B are timing charts for explaining a dimming controlmethod according to the second embodiment of the present invention;

[0053]FIG. 10 is a graph for explaining a dimming control method for animage reading apparatus according to the third embodiment of the presentinvention;

[0054]FIG. 11 is a graph in which the barycenter moving amount of animage read position in the dimming control method of the thirdembodiment of the present invention is compared with that in aconventional dimming control method;

[0055]FIGS. 12A to 12C are graphs for explaining the relationshipbetween the phase of a dimming signal and barycenter movement of theimage read position in the third embodiment of the present invention;

[0056]FIG. 13 is a block diagram showing the schematic arrangement ofthe control unit of an image reading apparatus according to the fourthembodiment of the present invention;

[0057]FIG. 14 is a block diagram showing details of the arrangement ofthe control unit shown in FIG. 13;

[0058]FIG. 15 is a block diagram showing details of a calculatorincluded in the control unit shown in FIG. 13;

[0059]FIG. 16 is a timing chart for explaining the operation of thecalculator shown in FIG. 15;

[0060]FIG. 17 is a block diagram showing details of a PWM signalgeneration unit included in the control unit shown in FIG. 13;

[0061]FIGS. 18A and 18B are timing charts for explaining a dimmingcontrol method according to the fifth embodiment of the presentinvention;

[0062]FIG. 19 is a block diagram showing details of the arrangement ofthe control unit of an image reading apparatus according to the sixthembodiment of the present invention;

[0063]FIG. 20 is a block diagram showing details of a PWM signalgeneration unit contained in the control unit shown in FIG. 19;

[0064]FIG. 21 is a timing chart for explaining the operation of the PWMsignal generation unit shown in FIG. 20;

[0065]FIG. 22 is a block diagram showing the schematic arrangement of acopying machine having, in the scanner unit, an image reading apparatusaccording to the seventh embodiment of the present invention;

[0066]FIG. 23 is a view showing the layout of line sensors included inthe scanner unit of the copying machine shown in FIG. 22;

[0067]FIG. 24 is a view showing an arrangement for phase-matching readsignals from the line sensors included in the scanner unit of thecopying machine shown in FIG. 22;

[0068]FIG. 25A is a view showing the layout of R, G, and B line sensorsemployed in the seventh embodiment of the present invention;

[0069]FIGS. 25B to 25D are views showing the color misregistrationamounts of the R, G, and B line sensors with the layout shown in FIG.25A;

[0070]FIG. 26A is a view showing a layout of R, G, and B line sensors,which is different from that shown in FIG. 25A;

[0071]FIGS. 26B to 26D are views showing the color misregistrationamounts of the R, G, and B line sensors with the layout shown in FIG.26A;

[0072]FIG. 27A is a view showing a layout of R, G, and B line sensors,which is different from that shown in FIG. 25A;

[0073]FIGS. 27B to 27D are views showing the color misregistrationamounts of the R, G, and B line sensors with the layout shown in FIG.27A;

[0074]FIG. 28 is a view for explaining color misregistration amountsbetween the different color components of read signals output from theR, G, and B line sensors;

[0075]FIG. 29 is a schematic view showing the optical system of ageneral image reading apparatus;

[0076]FIG. 30 is a graph for explaining a general dimming control methodusing pulse-width modulation;

[0077]FIG. 31 is a graph for explaining the afterglow characteristics ofR, G, and B light sources in the general dimming control method shown inFIG. 30; and

[0078]FIG. 32 is a graph for explaining the difference in barycenterposition of light-emitting amount between the R, G, and B light sourcesin the general dimming control method shown in FIG. 30.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0079] (First Embodiment)

[0080]FIGS. 1 and 2 are graphs for explaining fluorescent lamp lightingscheme of the first embodiment. in the embodiment, dimming control isperformed such that the center of the period (period of time) of dimmingby a control signal is placed at the center of a period Hsync, as shownin FIG. 1, unlike the conventional dimming control that is started insynchronism with the rise or fall of a periodical signal indicating theperiod Hsync (That is, one storage period of time of a solid-state imagesensing element), as shown in FIG. 30. In this case, even when the dutyof the control signal changes, the central position of the controlsignal remains at a center reference position C. For this reason, therise position of the control signal is controlled to be variable inaccordance with the duty.

[0081]FIG. 2 is a graph showing barycenter movement due to the afterglowcharacteristics in the control method of the first embodiment. When thelight-emitting region is set at the center of the period Hsync, as shownin FIG. 1, the afterglow amount is distributed to the front and rearsides of the light-emitting region and averaged. Barycenter movement dldue to afterglow is minimum and rarely influences the quality of a readimage.

[0082] A detailed arrangement for realizing the control method of thefirst embodiment will be described next. FIG. 3 is a perspective viewshowing an arrangement around a fluorescent lamp used as a light sourceof an image reading apparatus of the first embodiment.

[0083] As shown in FIG. 3, the two ends of a fluorescent lamp 1 aresupported by sockets 2 a and 2 b. A current is supplied from pins (notshown) of the sockets 2 a and 2 b. An aperture portion (optical opening)3 is formed in a predetermined region of the fluorescent lamp 1 to emitstrong light in the direction of an arrow a. A region other than theaperture portion 3 emits relatively weak light. A light amount sensor 4formed from a photodiode or the like is arranged at an appropriateportion of the fluorescent lamp 1 to detect a current corresponding tothe amount of light emitted by the fluorescent lamp 1. The light amountis controlled by a light amount controller in accordance with thedetected light-emitting amount to make the light amount of thefluorescent lamp almost constant.

[0084]FIG. 4 is a block diagram showing the light amount control circuitof the first embodiment. FIG. 5 is a block diagram showing details ofthe light amount control circuit shown in FIG. 4.

[0085] Referring to FIG. 5, an image processing unit 22 has a CCD(Charge-Coupled Device) 58 for receiving an optical signal from anoriginal 20 irradiated with the fluorescent lamp 1 and converting theoptical signal into an electrical signal corresponding to the lightamount, an analog processor 43 for performing predetermined signalprocessing for the electrical signal output from the CCD 58, and an A/Dconverter 30 for converting an analog signal output from the analogprocessor 43 into a digital signal. The CCD 58 stores charges read inone scanning period as one period of a sync signal. For this reason, anoutput signal from the CCD 58 has a magnitude obtained by integratingthe light amount in one scanning period. When lighting of thefluorescent lamp 1 and scanning by the CCD 58 are synchronized in thesame period, a predetermined output range can be obtained. An outputfrom the image processing unit 22 is sent to a printer and printed bythe printer.

[0086] On a mirror and light unit 21 for irradiating the original, thefluorescent lamp 1, a heater 33 attached to the fluorescent lamp 1, athermistor 34 attached to the heater 33 to detect the temperature of theheater, and the light amount sensor 4 are disposed. The light amountsensor 4 comprises a photodiode 35 for detecting the light amount of thefluorescent lamp 1 and outputting a light amount signal corresponding tothe light amount, and a preamplifier 36 for converting the small currentdetected by the photodiode 35 into a voltage signal.

[0087] The light amount signal output from the light amount sensor 4 isconverted into a voltage value and amplified by an amplifier 12. Thevoltage value amplified by the amplifier 12 is compared with apredetermined reference voltage by a comparator 13, and the comparisonresult is input to a light amount controller 14. To decrease the lightamount when the reflectance of a read image is especially high, thecomparator 13 operates a switch 38 on the basis of an instruction from aCPU 17 to switch the reference voltage.

[0088] A main-scanning sync signal SYNC1 generated by a sync signalgeneration circuit 16 is input to a delay adjustment circuit 18. Thedelay adjustment circuit 18 delays the sync signal SYNC1 by apredetermined delay amount in accordance with an instruction from theCPU 17 and outputs a sync signal SYNC2 to the light amount controller14.

[0089] On the basis of the output result of the comparator 13, the lightamount controller 14 outputs a pulse-width modulation (to be referred toas PWM hereinafter) PWM signal phase-locked with a predetermined syncsignal (Sync) which has been determined in advance, thereby controllingthe duty. The light amount controller 14 comprises a flip-flop (F/F)circuit 39 for outputting a light amount comparison signal from thecomparator 13, which is phase-locked with the sync signal SYNC2, anup/down counter 40 for incrementing/decrementing the count value insynchronism with the sync signal SYNC2 on the basis of the light amountcomparison signal, a down counter 41 for decrementing the output valuefrom the up/down counter 40 using a predetermined clock phase-lockedwith the sync signal to generate a PWM signal, and a preheat controlunit 42 for preheating the fluorescent lamp 1 before lighting.

[0090] The output value from the up/down counter 40 is input to the CPU17 to read the PWM value at an arbitrary timing. The CPU 17 calculates,in correspondence with the read PWM value, the delay amount of thefluorescent lamp control signal with respect to the periodical signalHsync, with which the center of the control signal matches the center ofthe period Hsync, and outputs the calculation result to the delayadjustment circuit 18. The calculation result from the CPU 17 is storedin a backup memory 10.

[0091] When the light amount is larger than a predetermined value, i.e.,the voltage value output from the amplifier 12 is larger the referencevoltage value, the output from the comparator 13, i.e., the output fromthe F/F 39 becomes zero, and the up/down counter 40 in the light amountcontroller 14 decrements the count value by a predetermined value. Asthe count value of the counter becomes small, the load value of the downcounter 41 decreases. With this operation, the pulse width of the PWMsignal to be input to an inverter 15 narrows to lower the duty ratio.Conversely, when the light amount is smaller than the predeterminedvalue, i.e., the voltage value output from the amplifier 12 is smallerthan the reference voltage value, the output from the comparator 13,i.e., the output from the F/F 39 is set at 1. The up/down counter 40increments the count value by a predetermined value, and the load valueof the down counter 41 is incremented. Hence, the pulse width of the PWMsignal to be input to the inverter 15 increases to make the duty ratiohigh. Upon powering on, the duty ratio of the PWM value is set at about100% to set the light-emitting amount of the fluorescent lamp 1 to havea predetermined value.

[0092] When the input PWM signal is at high level, the inverter 15supplies an AC current (ramp current) having a sufficiently higherfrequency (e.g., 10 to 100 times the frequency of the PWM signal) thanthat of the PWM signal to the fluorescent lamp 1 to turn on thefluorescent lamp 1. When the PWM signal is at low level, the rampcurrent is cut off to turn off the fluorescent lamp 1. The frequency ofthe PWM signal is higher than the optical frequency of the fluorescentlamp 1 in the ON or OFF state. Electrically, ON and OFF are repeated inaccordance with the period of the PWM signal. Apparently, thefluorescent lamp is turned on to emit light in a substantiallypredetermined amount corresponding to a current value obtained byaveraging the ramp current.

[0093]FIG. 6 shows a detailed arrangement of the delay adjustmentcircuit 18. This circuit comprises an up counter 48 for resetting themain-scanning sync signal SYNC1 and incrementing the count value inaccordance with a clock signal, comparators 49 and 50 for setting therise and fall coordinates of the comparators 48 and 50, and a JKF/F 53for receiving an output from the comparator 49 from the J input terminaland an output from the comparator 50 from the K input terminal andoutputting the sync signal SYNC2. For example, for an apparatus in whichthe number of pixels to be scanned in one main-scanning period is A (Ais a predetermined natural number), the rise coordinate is set on thebasis of A/2−duty value (%)/200×A, and the fall coordinate is set on thebasis of A/2−duty value (%)/200×A+1.

[0094]FIG. 7 is a block diagram of the down counter 41. The down counter41 comprises a down counter 57 and a JKF/F 55. The JKF/F 55 receives thesync signal SYNC1 from the J input terminal and a signal RC from thedown counter 57 from the K input terminal and outputs the PWM signal.Resetting is done for setting associated with fluorescent lamp controland canceled after regular fluorescent lamp control becomes possible.

[0095] Output signals on the circuits shown in FIGS. 4 and 5 will bedescribed next with reference to FIGS. 8A and 8B. Referring to FIGS. 8Aand 8B, the abscissa represents time, and the ordinate represents outputsignals. FIG. 8A shows output signals when the duty value is about 25%.FIG. 8B shows output signals when the duty value is about 60%. Sync1 isa sync signal output from the sync signal generation circuit 16 shown inFIG. 4. This signal is a reference signal for controlling the image readtiming, i.e., storage timing of the image sensing element. Sync2 is async signal obtained by delaying the sync signal Sync1 by the delayadjustment circuit 18 by a predetermined delay time T in accordance withan instruction from the CPU 17. The delay time input to the delayadjustment circuit 18 is calculated by the CPU 17 on the basis of theduty value output from the light amount controller 14. The method ofcalculating the delay time T will be described later.

[0096] Referring to FIG. 8A, the delay time from rise time t1 of thesync signal Sync1 to fall time t2 of the sync signal Sync2 isrepresented by A1. The delay time A1 can be calculated by the CPU 17 onthe basis of the duty value from the light amount controller 14 usingequation (1):

T=S×(100−duty)/2  . . . (1)

[0097] where T is the delay time, S is the Hsync period corresponding toone storage time, and duty is the duty value represented in percentage.The PWM signal as a light source driving pulse signal output from thelight amount controller 14 is output at fall time t2 of the delayed syncsignal Sync2, and the PWM signal of high level is continuously outputduring a period corresponding to the duty value. The inverter 15supplies a tube current having a frequency sufficiently higher than thatof the PWM signal to the fluorescent lamp 1 on the basis of the PWMsignal. In response to this tube current, the fluorescent lamp 1 isturned on to emit light in a substantially predetermined amountcorresponding to a current value as the average of the tube current. Atthis time, a line C at the center of the PWM signal, tube current, andlight amount signal in the ON state of the fluorescent lamp matches thecenter of the period of the sync signal Sync1 from the fall to the nextfall, that corresponds to one storage time of the solid-state imagesensing element.

[0098] Referring to FIG. 8B as well, the center C of the PWM signal,tube current, and light amount signal matches the center of theperiodical signal Sync1. In FIG. 8B, the duty value is about 60%. Adelay time B1 from fall time t3 of the sync signal Sync1 to fall time t4of the sync signal Sync2 is calculated from equation (1). When the dutyvalue becomes large, the delay time B1 become shorter than the delaytime A1 shown in FIG. 8A.

[0099] As described above, in this embodiment, the image read timing iscontrolled such that the central position of the ON control signal (PWMsignal) is always located almost at the center of the period of thesignal Hsync by shifting the phase of the sync signal Sync2 forcontrolling the timing of the PWM signal, i.e., the fluorescent lampON/OFF timing from that of the sync signal Sync1. With this arrangement,the central position of the ON control signal can be prevented fromchanging over time even when the duty value of the PWM signal ischanged. Even when the afterglow characteristics of phosphors change inunits of colors, the barycenter position of the light amount can alwaysbe located near the center of the periodical signal Hsync. Hence, thelight amount of afterglow in the OFF period can be averaged near the ONperiod in one storage time, and the change in barycenter position can beminimized.

[0100] (Second Embodiment)

[0101] A fluorescent lamp lighting method according to the secondembodiment of the present invention will be described with reference toFIGS. 9A and 9B. A detailed description of the same arrangements as inthe first embodiment will be omitted, and characteristic portions willbe mainly described.

[0102] Referring to FIGS. 9A and 9B, the abscissa represents time, andthe ordinate represents output signals. FIG. 9A shows output signalswhen the duty value is about 25%. FIG. 9B shows output signals when theduty value is about 60%.

[0103] In the second embodiment, the PWM signal in dimming changessymmetrically with respect to the center of the fall timing of a syncsignal Sync1. Sync1 represents a sync signal output from a sync signalgeneration circuit 16 of the present invention shown in the blockcircuit of FIG. 4 and controls the read timing, i.e., storage timing.Sync2 represents a sync signal obtained by delaying the sync signalSync1 by a delay adjustment circuit 18 in accordance with an instructionfrom a CPU 17 on the basis of the duty value from a light amountcontroller 14. The delay time from fall time t5 of the sync signal Sync1to fall time t6 of the sync signal Sync2 is represented by A2. The delaytime A2 can be calculated from equation (2) on the CPU 17 using the dutyvalue from the light amount controller 14.

T=S×(100−duty/2)  (2)

[0104] where T is the delay time, S is the Hsync period corresponding toone storage time, and duty is the duty value represented in percentage.The PWM signal output from the light amount controller 14 is output atfall time t6 of the delayed sync signal Sync2, and the PWM signal ofhigh level is continuously output during a period corresponding to theduty value. An inverter 15 supplies a tube current having a frequencysufficiently higher than that of the PWM signal to a fluorescent lamp 10on the basis of the PWM signal. In response to this tube current, thefluorescent lamp 10 is turned on to emit light in a substantiallypredetermined amount corresponding to a current value as the average ofthe tube current. At this time, time t5 at the center of the PWM signal,tube current, and light amount signal in the ON state of the fluorescentlamp matches the fall of the period of the sync signal Sync1, i.e., theread end timing (storage end timing), that corresponds to one storagetime of the solid-state image sensing element.

[0105] Referring to FIG. 9B as well, time t7 of the PWM signal, tubecurrent, and light amount signal matches the fall of the period of thesync signal Sync1, i.e., the read end timing (storage end timing), thatcorresponds to one storage time of the solid-state image sensingelement. In FIG. 9B, the duty value is about 60%, and a delay time B2from fall time t3 of the sync signal Sync1 to fall time t4 of the syncsignal Sync2 is calculated from equation (2). When the duty valuebecomes large, the delay time B2 becomes shorter than the delay time A2.

[0106] As described above, in the second embodiment, the centralposition of the ON control signal (PWM signal) can be prevented fromchanging over time and is always located at the fall time of theperiodical signal Hsync even when the duty value of the PWM signal ischanged. Even when the afterglow characteristics of phosphors change inunits of colors, the barycenter position of the light amount can belocated near the center of the period of the periodical signal Hsync.Hence, the light amount of afterglow in the OFF period can be averagednear the ON period in one storage time, and the change in barycenterposition can be minimized.

[0107] In the second embodiment, control is performed to locate thecentral position of the ON control signal at the fall timing of the syncsignal Sync1. However, the present invention is not limited to this, andcontrol may be performed to match the central position of the ON controlsignal with the rise of the sync signal Sync1, i.e., the read starttiming (storage start timing). If dimming of the light source isunnecessary, the duty value of the ON control signal need not always bechanged. A method other than the method described above may be used toreduce afterglow and suppress movement of the barycenter position foreach color due to the influence of afterglow. Even when a light sourceother than a fluorescent lamp is used, the shift of barycenter positiondue to afterglow can be decreased by the same method as described above.In addition, this embodiment can also be applied to a system whichsequentially turns on a plurality of light sources with colors.

[0108] (Third Embodiment)

[0109] A detailed description of the same arrangements as in the firstembodiment will be omitted, and characteristic portions will be mainlydescribed.

[0110]FIG. 10 is a graph for explaining the operation of an imagereading apparatus according to the third embodiment. The schematicarrangement of the optical system of this embodiment is the same as inFIG. 3, and a detailed description thereof will be omitted.

[0111] Referring to FIG. 10, the abscissa represents the dimming duty ofa dimming signal for dimming a light source such as a fluorescent lamp,and the ordinate represents the phase of the dimming signal in onestorage time of a solid-state image sensing element.

[0112] As the characteristic feature of the third embodiment, the phaseof the dimming signal in one storage time of the solid-state imagesensing element can be changed in accordance with the dimming duty (dutyvalue of the PWM signal) such that barycenter movement of the readposition for each color due to the influence of the afterglowcharacteristics of a phosphor, or the like is minimized.

[0113] As described above in the first and second embodiments, theinfluence of barycenter movement of the read position due to theafterglow characteristics of the light source such as a fluorescent lampcan be reduced by controlling the phase of the dimming signal andsetting the center of a light-emitting pulse corresponding to thefluorescent lamp driving pulse at the central position of one storagetime of the solid-state image sensing element.

[0114] However, if the phase of the dimming signal is only fixed at thecenter of a predetermined storage time (Sync1) of the solid-state imagesensing element, small barycenter movement may occur.

[0115] As shown in FIG. 11, barycenter movement of the read position dueto the dimming duty varies depending on the dimming duty amount.

[0116] As is apparent from FIG. 11, when the dimming duty is small,barycenter movement of the read position as the variation is large. Asthe dimming duty is close to 100%, the barycenter movement becomessmall.

[0117] This variation is improved when the phase of the dimming signalis generated symmetrically with respect to the center of one storagetime along the time axis, as shown in FIG. 11, though slight barycentermovement remains.

[0118]FIGS. 12A to 12C show the relationship between the phase of thedimming signal and barycenter movement of the image read position.

[0119]FIG. 12A shows barycenter movement of a light-emitting waveform ata predetermined duty in the dimming control method of generating adimming signal along the time axis symmetrically with respect to thecenter of one storage time of the solid-state image sensing element.

[0120] The duty value corresponds to D1 in FIG. 10. Since the control isperformed using the center of one storage time as a reference, the phaseof the dimming signal corresponds to A in FIG. 10. Referring to FIG.12A, the center of one storage time of the solid-state image sensingelement corresponds to a line D (alternate long and short dashed line).At this time, referring to FIG. 11, when the duty is represented by Dl,barycenter movement of the read position in dimming control for thecenter reference is represented by A, and barycenter movement of theread position remains, as shown in FIG. 12A.

[0121]FIG. 12B shows barycenter movement of a light-emitting waveformwhen the phase is slightly led with respect to the phase for the centerreference in FIG. 12. The phase of a dimming signal at this timecorresponds to B in FIG. 10.

[0122] When the phase of the dimming signal is led, the effect ofcontrol for the center reference decreases to close to the conventionaldimming control state described in “BACKGROUND OF THE INVENTION”. Hence,the residual barycenter movement amount becomes larger than that in FIG.12A.

[0123]FIG. 12C shows barycenter movement of a light-emitting waveformwhen the phase is slightly delayed with respect to the phase for thecenter reference in FIG. 12A. The phase of a dimming signal at this timecorresponds to C in FIG. 10.

[0124] Since the effect of control for the center reference is increasedby delaying the phase of the dimming signal, the residual barycentermovement amount can be made zero. More specifically, when the phase iscontrolled as shown in FIG. 12C, the residual barycenter movement amountcan be eliminated. By calculating the delay amounts of the dimmingsignal for the respective duties in advance, the residual barycentermovement amount can be made zero for any duty.

[0125] The curve in FIG. 10 represents the delay amount with which thebarycenter movement amount becomes zero for the respective duties. Thesmaller the dimming duty is, the larger the delay amount from the phaseas the center reference becomes.

[0126] The delay amount from the phase as the center reference for eachduty is obtained in advance by calculation or measurement. When thiscorrespondence is stored on a storage medium such as a backup memory 29of the image reading apparatus in advance, correction becomes easy.

[0127] An arrangement for realizing the control scheme of thisembodiment will be described next.

[0128] The third embodiment is different from the first embodiment inthe following points.

[0129] For example, for an apparatus in which the number of pixels to bescanned in one main scanning period is A (A is a predetermined naturalnumber), the rise coordinate is set on the basis of

[0130] A/2−duty value (%)/200×A+B(duty)

[0131] The fall coordinate is set on the basis of

[0132] A/2−duty value (%)/200×A+B(duty)+1

[0133] where B(duty) is an arbitrary delay amount determined by theduty. This value is determined by the number of pixels to be shiftedfrom the center of one storage time to minimize barycenter movement ofthe read position.

[0134] A delay time T is given by

T=S1×(100−duty value)/2+S2  (3)

[0135] where T is the delay time, S1 is a period Hsync corresponding toone storage time, duty is the duty value represented in percentage, andS2 is an arbitrary delay amount set in correspondence with the duty.

[0136] The delay amounts to be employed as B(duty) and S2 are stored inthe backup memory 29 or the like in advance in correspondence with dutyvalues, and a delay amount corresponding to the current duty is read outfrom the stored values.

[0137] When the control method of the third embodiment is employed,barycenter movement of the read position can be further reduced ascompared to the first embodiment.

[0138] (Fourth Embodiment)

[0139] In the fourth embodiment, pulse output delay control described inthe above third embodiment is applied to the image reading apparatusdescribed in the second embodiment.

[0140] In this case, a delay time T is given by

T=S1×(100−duty value/2)+S2  (4)

[0141] where T is the delay time, S1 is the period Hsync correspondingto one storage time, duty is the duty value represented in percentage,and S2 is an arbitrary delay amount set in correspondence with the duty.

[0142] By employing delay control with such calculation, the barycenterposition change amount can be further reduced as compared to the secondembodiment.

[0143] (Fifth Embodiment)

[0144]FIG. 13 is a block diagram of a light amount control systemaccording to the fourth embodiment.

[0145] A light amount sensor 511 detects the light amount of afluorescent lamp 510 and outputs a light amount signal corresponding tothe light amount. The light amount is converted into a voltage value andamplified by an amplifier 512.

[0146] The amplified voltage value is compared with a predeterminedreference voltage by a comparator 513. The comparison result is input toa light amount controller 514.

[0147] The light amount controller 514 outputs a pulsewidth modulation(to be referred to as PWM hereinafter) signal as shown in FIG. 1 to aninverter 515, which is phase-locked with a sync signal (Sync), therebyperforming duty control. More specifically, when the voltage valueoutput from the amplifier 512 is larger than the reference voltagevalue, a PWM signal is output to lower the duty ratio. Conversely, whenthe voltage value output from the amplifier 512 is smaller than thereference voltage value, a PWM signal is output to raise the duty ratio.A duty value to be set in the inverter 515 is output from the lightamount controller 514 to a CPU 517.

[0148] When the PWM signal input to the inverter 515 is at high level,the inverter 515 supplies an AC current, i.e., a ramp current having afrequency sufficiently higher (e.g., frequency of 10 to 100 times thefrequency of the PWM signal) than that of the PWM signal to thefluorescent lamp 510 to turn on the fluorescent lamp 510. When the PWMsignal is at low level, the inverter 515 cuts off the ramp current toturn off the fluorescent lamp 510.

[0149] The frequency of the PWM signal is higher than the opticalfrequency of the fluorescent lamp 510 in the ON or OFF state.Electrically, ON and OFF are repeated in accordance with the period ofthe PWM signal. Apparently, the fluorescent lamp is turned on to emitlight in a substantially predetermined amount corresponding to a currentvalue obtained by averaging the ramp current.

[0150] The arrangement of an image reading apparatus according to thefifth embodiment, which uses the above light amount control arrangement,will be described below.

[0151]FIG. 14 is a block diagram showing the arrangement of the imagereading apparatus according to the fifth embodiment. As shown in FIG.14, the apparatus comprises a mirror and light unit 521 for irradiatingan original 520 with light, an image processing unit 522 for performingpredetermined image processing for an optical signal from the original520 and outputting the signal to a printer, an amplifier 524 foramplifying the output signal from the mirror and light unit 521, acomparator 525 for comparing the output signal from the amplifier 524with a reference signal and outputting the comparison result, a lightamount controller 526 formed from, e.g., an ASIC for controlling thelight amount on the basis of the output result from the comparator 525and outputting a PWM signal phase-locked with a predetermined syncsignal, an inverter 527 for turning on the lamp on the basis of aninstruction from the light amount controller 526, a CPU 528 forcontrolling the entire apparatus, and a backup memory 529 for storing acalculation result from the CPU 528, and the like. Reference numeral 544denotes an A/D converter; and 545, a circuit for generating afree-running main scanning sync signal (SYNC) and selecting forselecting one of the main scanning sync signal and a printer mainscanning sync signal BD.

[0152] The mirror and light unit 521 comprises a fluorescent lamp 532, aheater 533 attached to the fluorescent lamp 532, a photodiode 535attached to the fluorescent lamp 532 to detect the light amount of thefluorescent lamp 532, and a light amount sensor 537 having thephotodiode 535 and a preamplifier 536 for converting a microcurrentdetected by the photodiode 535 into a voltage signal.

[0153] The amplifier 524 receives a voltage signal output from thepreamplifier 536 and a voltage signal from a rheostat 523 and amplifiesthe light amount signal to a predetermined value.

[0154] To reduce the light amount when the reflectance of the read imageis particularly high, the comparator 525 initialize a switch 538 on thebasis of an instruction from the CPU 528 to allow switching thereference voltage.

[0155] The light amount controller 526 comprises a flip-flop (F/F)circuit 539 for outputting a light amount comparison signal from thecomparator 525, which is phase-locked with a sync signal, a PWM widthdetermination unit 540 for incrementing/decrementing a counter value inaccordance with the light amount comparison signal and sync signal(SYNC1) to determine the PWM width, a PWM signal generation unit 541 foroutputting a PWM signal having a PWM width determined at a predeterminedposition and phase-locked with a sync signal (SYNC1), a calculator 561for obtaining a set value to be set in the PWM signal generation unit541 using the output value, i.e., the PWM width value from the PWM widthdetermination unit 540, and a preheat control unit 542 for preheatingthe fluorescent lamp 532 before it is turned on.

[0156] The PWM width determination unit 540 is formed from an up/downcounter. When the voltage value input to the comparator 525 is largerthan the reference voltage value, the count value is decremented to makethe count value small, i.e., the duty ratio low. When the voltage valueinput to the comparator 525 is smaller than the reference voltage value,the count value is incremented to make the count value large, i.e., theduty ratio high.

[0157] The output value from the PWM width determination unit 540 isinput to the CPU 528. The CPU 528 reads the PWM value at an arbitrarytiming.

[0158] The operation of the light amount controller 526 will bedescribed. When the light amount is larger than a predetermined value,the output from the comparator 525, i.e., the output from the F/F 539 isset at 0. The output from the PWM width determination unit 540 isdecreased by a predetermined value, and consequently, the PWM signal(pulse width) to be input to the inverter 527 is narrowed. Conversely,when the output value is smaller than the predetermined value, theoutput from the comparator 525, i.e., the output from the F/F 539 is setat 1. The output from the PWM width determination unit 540 is increasedby a predetermined value to increase the PWM width value. As a result,the PWM value (pulse width) to be input to the inverter 527 isincreased.

[0159] Upon powering on, the duty ratio is set at about 100% to causethe fluorescent lamp 510 to emit light in a predetermined light amount.

[0160] When the PWM signal input to the inverter 527 is at high level,the inverter 527 supplies an AC current (ramp current) having asufficiently higher frequency (e.g., 10 to 100 times the frequency ofthe PWM signal) than that of the PWM signal to the fluorescent lamp 532to turn on the fluorescent lamp 532. When the PWM signal is at lowlevel, the ramp current is cut off to turn off the fluorescent lamp 532.Electrically, ON and OFF are repeated in accordance with the period ofthe PWM signal. Apparently, the fluorescent lamp is turned on to emitlight in a substantially predetermined amount corresponding to a currentvalue obtained by averaging the ramp current.

[0161] The image processing unit 522 has a CCD 558 for receiving anoptical signal from the original 520 and converting the signal into anelectrical signal, an analog processor 543 for receiving the electricalsignal output from the CCD 558 and performing predetermined signalprocessing, and the A/D converter 544 for converting the analog signaloutput from the analog processor 543 into a digital signal. The CCD 558stores charges read in one scanning period as one period of a syncsignal. Hence, an output signal from the CCD 558 has a magnitudeobtained by integrating the light amount in one scanning period. Whenlighting of the fluorescent lamp 532 and scanning by the CCD 558 aresynchronized in the same period, a predetermined output can be obtained.

[0162]FIG. 15 is a block diagram of the calculator 561. FIG. 16 is atiming chart of the operation of the calculator. The timing chart shownin FIG. 16 will be described. A desired pulse width (t) is obtained bythe PWM width determination unit 540. As described above, the pulse mustbe located at the center of one sync signal (one SYNC period: time T).For this purpose, the calculator 561 calculates

L1=T/2−t/2

L2=L1+t

[0163] and outputs the results, L1 and L2.

[0164]FIG. 17 is a block diagram of the PWM signal generation unit 541.

[0165] Referring to FIG. 17, reference numeral 548 denotes an upcounter; 549 and 550, comparators, and 553, a JK flip-flop.

[0166] The counter 548 is reset by a main scanning sync signal 547(SYNC) from the image processing unit 22 or the like and incremented inaccordance with a clock signal. The comparators 549 and 550 determinethe rise and fall timings. The JK flip-flop 553 generates a PWM signalin accordance with outputs from the comparators 549 and 550. The outputvalues L1 and L2 from the calculator 561 are set in the comparators 549and 550, respectively.

[0167] Output signals on the circuit shown in the block diagram of FIG.14, that are obtained by such a control scheme, will be described withreference to FIGS. 18A and 18B.

[0168] As output signals, a Sync signal, a PWM signal, a control currentwaveform (tube current), and a light amount will be described.

[0169] Referring to FIGS. 18A and 18B, the abscissa represents time, andthe ordinate represents the output signals.

[0170]FIG. 18A shows the output signals when the duty ratio is about25%. FIG. 18B shows the output signals when the duty ratio is about 60%.

[0171] Sync1 represents a Sync signal output from a Sync generator 516shown in the block diagram of FIG. 13.

[0172] The PWM signal of high level is output from the light amountcontroller 514 continuously during a period corresponding to the dutyvalue.

[0173] On the basis of the PWM signal, the inverter 527 supplies acurrent having a frequency sufficiently higher than that of the PWMsignal to the fluorescent lamp 510.

[0174] Referring to FIGS. 18A and 18B, the tube current represents acurrent signal output from the inverter 527. In response to this tubecurrent, the fluorescent lamp 510 is turned on to emit light in asubstantially predetermined amount corresponding to a current valueobtained by averaging the tube current.

[0175] A line C at the center of the PWM signal, tube current, and lightamount signal in the ON state of the fluorescent lamp matches the centerof the period of the sync signal Sync1 from the fall timing to the nextfall timing, that corresponds to one storage time of a solid-state imagesensing element 58.

[0176] Referring to FIG. 18B as well, the center C of the PWM signal,tube current, and light amount signal matches the center of theperiodical signal Sync1. In FIG. 18B, the duty ratio is about 60%. Evenwhen the duty ratio changes, the central position of the ON controlsignal does not change over time and is located at the center C of theperiodical signal Hsync. Even when the afterglow characteristics ofphosphors change in units of colors, the barycenter position of thelight amount can always be located near the center of the periodicalsignal Hsync. Hence, the light amount of afterglow in the OFF period canbe averaged near the ON period in one storage time, and the change inbarycenter position can be minimized.

[0177] As described above, according to the fifth embodiment, when anoriginal image is to be read using a white light source having phosphorsof a plurality of colors, which have different afterglowcharacteristics, barycenter movement of the read position in thesub-scanning direction, which occurs in units of colors depending on theafterglow characteristics of the light source, can be reduced orcorrected. More specifically, pulse-width modulation is used as a lightamount control means for the light source, and a control pulse isgenerated symmetrically with respect to the center of the periodicalsignal Hsync along the time axis. With this arrangement, even when theafterglow characteristics of colors contained in the light sourcechange, the barycenter position of the light amount can always belocated near the center of the periodical signal Hsync. Hence, the lightamount of afterglow in the OFF period can be averaged near the ON periodin one storage time, the change in barycenter position can be minimized,and color misregistration in reading in the sub-scanning direction canbe reduced.

[0178] (Sixth Embodiment)

[0179]FIG. 19 is a block diagram showing the arrangement of an imagereading apparatus according to the sixth embodiment.

[0180] Referring to FIG. 19, a light amount controller 526 comprises aflip-flop circuit 539 for outputting a light amount comparison signalfrom a comparator 525, which is phase-locked with a sync signal, a PWMwidth determination unit 540 incrementing/decrementing the counter insynchronism with a sync signal (SYNC1) on the basis of the light amountcomparison signal and determining the PWM width, a PWM signal generationunit 572 for outputting a PWM signal having a PWM width determined at adesired position and phase-locked with the sync signal (SYNC1), a PWMposition determination unit 571 for determining the position of the PWMsignal, and a preheat control unit 542 for preheating a fluorescent lamp532 before it is turned on.

[0181]FIG. 20 is a block diagram showing the arrangement of the PWMsignal generation unit 572.

[0182] Referring to FIG. 20, reference numeral 548 denotes an upcounter; 549 and 550, comparators; 553, a JK flip-flop, and 555, anadder.

[0183] The counter 548 is reset by a main scanning sync signal 547(SYNC1) from an image processing unit 22, or the like, and incrementedin accordance with a clock signal. The adder 555 adds an output t fromthe PWM width setting unit 540 and an output L3 from the PWM positiondetermination unit 571. The comparators 549 and 550 determine the riseand fall timings of a control pulse. The JK flip-flop 553 generates aPWM signal in accordance with outputs from the comparators 549 and 550The output L3 from the PWM position determination unit 571 and an outputfrom the adder 555 are set in the comparators 549 and 550, respectively.

[0184] The operation of this embodiment will be described with referenceto the timing chart shown in FIG. 21. The output from the PWM signalgeneration unit 572 need be set at the center of one sync signal (oneSYNC period: time T). For this purpose, the time (L3 in FIG. 21) fromthe end of the PWM signal to the start of the next SYNC period isregarded as the PWM start time (L4 in FIG. 21) of the next SYNC period.

[0185] Hence, the PWM position determination unit 571 is formed from acounter for obtaining the time L3 from the output from the PWM signalgeneration unit 572 and the sync signal (SYNC). The output from the PWMposition determination unit 571 corresponds to the time L3. By inputtingthe time L3 to the PWM signal generation unit 572, as shown in FIG. 20,a desired PWM signal can be obtained.

[0186] According to the sixth embodiment, the control pulse is generatedsymmetrically with respect to the center of the storage time T along thetime axis, and the same effect as in the first embodiment can beobtained.

[0187] When the average value of L3 in a predetermined time is input tothe comparator 549 and adder 555 instead of directly inputting theoutput L3 from the PWM position determination unit 571 to the comparator549 and adder 555, the variation in phase of the control pulse can besubstantially eliminated.

[0188] (Seventh Embodiment)

[0189]FIG. 22 is a block diagram showing the arrangement of principalpart of a copying machine having an image reading apparatus according tothe seventh embodiment.

[0190] Referring to FIG. 22, a scanner unit 601 reads an original placedon an original table. As a rod-shaped light source, a white fluorescentlamp in which phosphors corresponding to the R, G, and B colors aremixed and applied is used. As photoelectric conversion elements, threeCCD line sensors of R, G, and B colors are used. These line sensors havea general structure as in the optical system shown in FIG. 29.

[0191]FIG. 23 shows the layout of the three line sensors. CCD linesensors 621, 622, and 623 are laid out with an offset corresponding to 8lines therebetween and sequentially read an original image on theoriginal table by scanning the same mirror optical system as in FIG. 29.

[0192] Since the CCD line sensors 621, 622, and 623 are laid out with anoffset of 8 lines therebetween, they read the original image atpositions shifted by 8 lines at the same time.

[0193] The original image read positions at the same time will becompared. The read position of the G CCD line sensor 623 is mostadvanced. With respect to the read position of the G CCD line sensor623, the read position of the B CCD line sensor 622 is delayed by 8lines, and that of the R CCD line sensor 621 is delayed by 16 lines.

[0194]FIG. 24 shows a block for correcting the different read positions.

[0195] Referring to FIG. 24, reference numeral 625 denotes a 16-linememory for delaying image signals of 16 lines; and 626, an 8-line memoryfor delaying image signals of 8 lines. An output from the G CCD linesensor 623 is input to the 16-line memory 625 and delayed by 16 lines.The signal delayed by 16 lines is output from the 16-line memory 625.

[0196] As described above, the G CCD line sensor 623 is laid out at aposition offset from the R CCD line sensor 621 by 16 lines. As outputsfrom the 16-line memory 625, the read position of the output from the GCCD line sensor and that of the output from the R CCD line sensorcorrespond to the same line on the original.

[0197] Similarly, an output signal from the B CCD line sensor 622 isdelayed by 8 lines by the 8-line memory 626. As outputs from the 8-linememory 626, the read position of the output from the B CCD line sensor622 and that of the output from the G CCD line sensor 621 correspond tothe same line on the original.

[0198] As described above, the shift in read position between the threeCCD line sensors 621, 622, and 623 for R, G, and B components, which arelaid out with an offset, is corrected by the block shown in FIG. 24.Hence, as outputs from the block shown in FIG. 24, R, G, and B signalscorresponding to the same position on the original image can beobtained.

[0199] The output signals shown in FIG. 24 are subjected to well-knownshading correction to correct a light-emitting variation of thefluorescent lamp and output from the scanner unit 601. An input maskingunit 602 corrects the color balance between the R, G, and B signalsoutput from the scanner unit 601. The input masking unit 602 performsmasking processing using well-known 3×3 matrix calculation below. Rout

Rout=K00×Rin+K01×Gin+K02×Bin

Gout=K10×Rin+K11×Gin+K12×Bin

Bout=K20×Rin+K21×Gin+K22×Bin

[0200] where K00 to K22 are constants.

[0201] Output signals from the input masking unit 602 are input to a LOGconversion unit 603.

[0202] The R, G, and B signals input to the LOG conversion unit 603 areconverted into Y, M and C density signals by well-known logarithmictransformation and input to an output masking unit 604. The outputmasking unit 604 performs masking correction processing for the receivedY, M and C signals in consideration of the characteristics of colormaterials used for printing and printing characteristics of the printer.With this processing, Y, M, C and K signals are generated, and thegenerated surface-sequential signals are output.

[0203] However, in this embodiment, one of the Y, M, C, and K signals isselectively output from the output masking unit 604. Every time thescanner unit 601 scans the original, the signal output from the outputmasking unit 604 is switched between the Y, M, C, and K signals. Everytime the scanner unit 601 scans the original, an image signalcorresponding to a color material of yellow, magenta, cyan, and black isoutput. When the scanner unit 601 scans one original four times, afull-color print image is output from a printer unit 606. The outputsignal from the output masking unit 604 is input to an output conversionunit 605. The output conversion unit 605 corrects the density signallevel (γ correction) in accordance with the gradation characteristics ofthe printer, converts the input 8 bit multilevel Y, M, C, and K signalsinto 1-bit binary signals by pseudo halftone processing, and outputs thesignals to the printer unit 606. The printer unit 606 controls printingon a printing paper sheet in accordance with the 1-bit Y, M, C, and Ksignals output from the output conversion unit 605 to form a printedimage. A control unit 607 controls the entire apparatus, i.e., controlsthe operation of the apparatus in accordance with an input from anoperation unit (not shown).

[0204] A barycenter shift in read position in the sub-scanningdirection, that is generated due to the difference in attenuationcharacteristics between the R, G, and B colors when a white fluorescentlamp in which phosphors corresponding to the R, G, and B colors aremixed and applied to the inner wall of the tube is used as a lightsource will be described below in detail.

[0205] The phenomenon that the barycenter of the read position in thesub-scanning direction changes depending on the color occurs due to thedifference in read position between the CCD line sensors 621, 622, and623 of the R, G, B colors. This will be described with reference toFIGS. 25A to 25D.

[0206]FIG. 25A is a view showing the layout of the CCD line sensors 621,622, and 623 of R, G, and B colors. Referring to FIG. 25A, the CCD linesensors 621, 622, and 623 are physically laid out at an interval of 8lines. The phase relationship between the original image read signalsoutput from the CCD line sensors is represented by adding, to thephysical interval of the layout of the CCD line sensors, i.e., 8 lines,the difference in barycenter position, that is generated by thedifference in attenuation characteristics between the R, G, and B colorsin the light-emitting amount of the fluorescent lamp. The phasedifferences between the R, G, and B read signals, including thedifference in barycenter position, can be represented as follows.

[0207] The phase difference between the B read signal and R read signal

=8+(Kb−Kr), Kb−Kr<0

[0208] The phase difference between the G read signal and B read signal

=8+(Kg−Kb), Kg−Kb>0

[0209] The phase difference between the G read signal and R read signal

=8+(Kg−Kr), Kg−Kr≈0

[0210] where Kr, Kg, and Kb are the barycenter movement distances of theR, G, and B colors in the light-emitting amounts. The values Kr and Kgare larger than the value Kb. The values Kr and Kg almost equal.

[0211] This yields

|Kb−Kg|>|Kb−Kr|>|Kg−Kr|

[0212] As described above, because of the differences between the colorbarycenter positions, the phase difference between the B read signal andR read signal is smaller than 8 lines. The phase difference between theG, and B read signals is larger than 8 lines. The phases of the G readsignal and R read signal are almost equal to 16 lines.

[0213] As for correction of the read positions of the CCD line sensors621, 622, and 623 of R, G, and B colors, the phases of the read signalsof different colors are set in order by the arrangement shown in FIG.24, so the phase difference between read positions, that is larger than8 lines, is a steady offset component.

[0214] This state is shown in FIGS. 25B to 25D as color misregistrationamounts between the colors. A color misregistration amount is the amountof shift in read position between the colors, that is generated at theedge portion between white and black portions when an original imagewith a black and white repeating pattern as shown in FIG. 28 is read.

[0215] Vibration components in FIGS. 25A to 25D are components generatedwhen the read positions of the CCD line sensors 621, 622, and 623 of therespective colors shift due to the influence of vibration in the mirroroptical system when it scans. For example, the vibration component ofthe G−R color misregistration amount in FIG. 25B is generated by thedifference in the magnitude and direction of vibration at a positionseparated by 16 lines during scanning by the mirror optical system.

[0216] Generally, the vibration period of the mirror optical system islonger than the interval between the CCD line sensors of the respectivecolors. For this reason, the vibration component between colors with thelargest interval becomes larger than the vibration components betweenthe remaining colors. Let D be the vibration component of the G−R colormisregistration amount. The B−R and G−B color misregistration vibrationcomponents are 1/2 the G−R color misregistration vibration component andrepresented by D/2.

[0217] The average value of color misregistration amounts between thecolors corresponds to a component generated by the above-describeddifference between barycenter positions of the light-emitting amountsand appears as a color misregistration offset component. The amount ofthe color misregistration vibration component D is larger than the colormisregistration amount offset amount generated by the difference betweenbarycenter positions of the light-emitting amounts. A maximum colormisregistration amount M1 in the CCD layout shown in FIG. 25A isgenerated by the G−R color misregistration and given by

M1=|Kg−Kr|+D/2

[0218] As a reference, other layouts of the R, G, and B CCD line sensors621, 622, and 623 are shown in FIGS. 26A to 26D and 27A to 27D.

[0219] In these examples, the B CCD line sensor that has the minimumamount of barycenter movement in the light-emitting amount is laid outnot at the center but at the end portion.

[0220] In the layout shown in FIG. 26A, the color misregistration offsetcomponent generated by barycenter movement in the light-emitting amountis maximized between B and G colors, as shown in FIG. 26D. As shown inFIG. 26B, the B−R color misregistration offset component substantiallyequals the B−G color misregistration offset component.

[0221] The absolute value |Kg−Kr| of the difference between the B−Rcolor misregistration offset component Kb−Kr and B−G colormisregistration amount offset component Kb−Kg is sufficiently smallerthan the value of the vibration component D/2.

[0222] Hence, the B−R color misregistration generated by the colormisregistration amount vibration component D due to the vibration factorof the mirror optical system has a maximum color misregistration amountM2. The maximum color misregistration amount M2 is given by

M2=|Kb−Kr|+D/2

[0223] Similarly, a maximum color misregistration amount M3 in the CCDlayout shown in FIG. 27A is the G−B color misregistration amount and isgiven by:

M3=|Kg−Kb|+D/2

[0224] The maximum color misregistration amounts in the above-describedlayouts of CCDs of the colors will be compared. Because of thebarycenter movement amount in the light-emitting amount,

|Kg−Kr|<|Kb−Kr|<|Kg−Kb|

[0225] The following relationship holds between the maximum colormisregistration amounts.

M1<M2<M3

[0226] That is, when a CCD line sensor corresponding to a color with alarge difference in barycenter movement in the light-emitting amount isnot located at an end, as shown in FIG. 25A, the maximum colormisregistration amount can be minimized.

[0227] In this embodiment, the afterglow characteristics of the lightsource are smallest for the B color component and larger for the R and Gcolor components. However, the afterglow characteristics of the lightsource change depending on the materials of the phosphors used. Whenline sensors corresponding to two colors with the largest difference incharacteristics are not laid out at two ends, the color misregistrationamount between the two colors at the two ends can be reduced, as isapparent from the description of the above embodiment. In thisembodiment, the three line sensors are laid out at an interval of 8lines. However, the present invention is not limited to this.

[0228] As described above, according to the seventh embodiment, when awhite light source having phosphors with different afterglowcharacteristics in units of read colors corresponding to the R, G, and Bline sensors is used, the layout of the R, G, and B line sensors isdetermined in consideration of the magnitudes of the afterglowcharacteristics. More specifically, line sensors corresponding to twocolors with the largest difference in afterglow characteristics are notlaid out at two ends. With this arrangement, the influence of readposition shift between the colors due to read position barycentermovement in the sub-scanning direction, that is generated by thedifference in the afterglow characteristics of the light source, can beprevented from conspicuously appearing in read signals, and degradationin image quality due to color misregistration can be minimized.

[0229] In the above-described embodiments, a fluorescent lamp foremitting white light containing R, G, and B color components is used asa light source. However, the present invention is not limited to thisand can also be applied to an image reading apparatus using, as a lightsource, fluorescent lamps of a plurality of colors, which are turned onto generate pseudo white light. In this case, a driving pulse isgenerated symmetrically with respect to the reference position along thetime axis in accordance with an increase in dimming duty. In the aboveembodiments, a reflective original is read. However, the presentinvention is not limited to this, and a transparent original may beread.

[0230] As many apparently widely different embodiments of the presentinvention can be made without departing from the spirit and scopethereof, it is to be understood that the invention is not limited to thespecific embodiments thereof except as defined in the appended claims.

What is claimed is:
 1. An image reading apparatus for irradiating anoriginal image with a light source and forming an image corresponding tothe original image on an image sensing device through an imaging opticalsystem to read the original image, comprising: control means forshifting an ON start timing of the light source for illuminating theoriginal image from a start timing of a predetermined charge storageperiod of the image sensing device.
 2. The apparatus according to claim1, wherein said control means controls a phase of a control pulse fordriving the light source so as to shift the ON start timing of the lightsource.
 3. The apparatus according to claim 2, wherein said controlmeans generates the control pulse symmetrically with respect to apredetermined reference timing in the charge storage period along a timeaxis.
 4. The apparatus according to claim 3, wherein said control meansuses the reference timing as a central position of the charge storageperiod along the time axis.
 5. The apparatus according to claim 3,wherein said control means uses the reference timing as a start timingor an end timing of the charge storage period.
 6. The apparatusaccording to claim 3, wherein said control means determines a starttiming or an end timing of the control pulse in accordance with a dutyratio of the control pulse, that is determined by pulse-width modulationin accordance with the charge storage period and a magnitude of anoutput signal from the image sensing device, so as to generate thecontrol pulse substantially symmetrically with respect to the referencetiming along the time axis.
 7. The apparatus according to claim 1,wherein the light source contains a plurality of color components. 8.The apparatus according to claim 7, wherein said control means causesthe light source to emit the plurality of color components in accordancewith the same control pulse.
 9. The apparatus according to claim 7,wherein the plurality of color components have afterglow characteristicsdifferent from each other.
 10. The apparatus according to claim 1,wherein the light source comprises a fluorescent lamp.
 11. The apparatusaccording to claim 10, wherein a plurality of phosphors applied to aninner wall of a tube of the fluorescent lamp have afterglowcharacteristics different from each other.
 12. The apparatus accordingto claim 1, wherein the image sensing device comprises a plurality ofline sensors for reading images of different color components.
 13. Theapparatus according to claim 1, wherein said control means controls aduty ratio of a control pulse by pulse-width modulation.
 14. Theapparatus according to claim 13, further comprising a memory storingrelationships between phases and duty ratios of the control pulse, andwherein said control means, in controlling the phase of the controlpulse for driving the light source, adjusts the phase of the controlpulse with reference to said memory in accordance with the duty ratio ofthe control pulse, which is determined by pulse-width modulation.
 15. Animage reading apparatus for irradiating an original image with a lightsource and forming an image corresponding to the original image on animage sensing device through an imaging optical system while scanningthe original image in main and sub-scanning directions so as to read theoriginal image, comprising: control means for reducing barycentermovement of read positions of a plurality of color components in thesub-scanning direction, that is generated by afterglow characteristicsof the plurality of color components contained in the light source forilluminating the original image.
 16. The apparatus according to claim15, wherein said control means controls a phase of a control pulse fordriving the light source so as to shift the ON start timing of the lightsource.
 17. The apparatus according to claim 16, wherein said controlmeans generates the control pulse symmetrically with respect to apredetermined reference timing in the charge storage period along a timeaxis.
 18. The apparatus according to claim 17, wherein said controlmeans uses the reference timing as a central position of the chargestorage period along the time axis.
 19. The apparatus according to claim17, wherein said control means uses the reference timing as a starttiming or an end timing of the charge storage period.
 20. The apparatusaccording to claim 17, wherein said control means determines a starttiming or an end timing of the control pulse in accordance with a dutyratio of the control pulse, that is determined by pulse-width modulationin accordance with the charge storage period and a magnitude of anoutput signal from the image sensing device, so as to generate thecontrol pulse substantially symmetrically with respect to the referencetiming along the time axis.
 21. The apparatus according to claim 15,wherein the light source contains a plurality of color components. 22.The apparatus according to claim 21, wherein said control means causesthe light source to emit the plurality of color components in accordancewith the same control pulse.
 23. The apparatus according to claim 21,wherein the plurality of color components have afterglow characteristicsdifferent from each other.
 24. The apparatus according to claim 15,wherein the light source comprises a fluorescent lamp.
 25. The apparatusaccording to claim 24, wherein a plurality of phosphors applied to aninner wall of a tube of the fluorescent lamp have afterglowcharacteristics different from each other.
 26. The apparatus accordingto claim 15, wherein the image sensing device comprises a plurality ofline sensors for reading images of different color components.
 27. Theapparatus according to claim 15, wherein said control means controls aduty ratio of a control pulse by pulse-width modulation.
 28. Theapparatus according to claim 27, further comprising a memory storingrelationships between phases and duty ratios of the control pulse, andwherein said control means, in controlling the phase of the controlpulse for driving the light source, adjusts the phase of the controlpulse with reference to said memory in accordance with the duty ratio ofthe control pulse, which is determined by pulse-width modulation.
 29. Animage reading apparatus having a white light source containing R (red),G (green), and B (blue) color components with afterglow characteristicsdifferent from each other, and line sensors of R, G, and B colors, whichare laid out with an offset in a sub-scanning direction, wherein therelative layout of the line sensors of R, G, and B colors is determinedon the basis of the afterglow characteristics of the R, G, and B colorcomponents of the white light source.
 30. The apparatus according toclaim 29, wherein line sensors corresponding to two of the R, G, and Bcolor components of the white light source, which have the largestdifference in afterglow characteristics, are not laid out at two ends.31. The apparatus according to claim 30, wherein line sensorscorresponding to the R and G color components are laid out at two ends,and a line sensor corresponding to the B color component is laid out ata center.
 32. The apparatus according to claim 29, further comprising: asensor for detecting a light amount of the white light source; anddimming control means for controlling a current to be supplied to thewhite light source so as to hold the light amount detected by saidsensor at a predetermined value.
 33. The apparatus according to claim32, wherein a control period of said dimming control means is insynchronism with a predetermined charge storage period of the linesensor.
 34. The apparatus according to claim 29, wherein the white lightsource contains phosphors having afterglow characteristics differentfrom each other and corresponding to the R, G, and B color components.35. A dimming control method for a light source in an image readingapparatus for irradiating an original image with the light source andforming an image corresponding to the original image on an image sensingdevice through an imaging optical system to read the original image,comprising: a control step of shifting an ON start timing of the lightsource for illuminating the original image from a start timing of apredetermined charge storage period of the image sensing device.
 36. Themethod according to claim 35, wherein, in the control step, a phase of acontrol pulse for driving the light source is controlled so as to shiftthe ON start timing of the light source.
 37. The method according toclaim 36, wherein, in the control step, the control pulse is generatedsymmetrically with respect to a predetermined reference timing in thecharge storage period along a time axis.
 38. The method according toclaim 37, wherein, in the control step, the reference timing is set at acentral position of the charge storage period along the time axis. 39.The method according to claim 37, wherein, in the control step, thereference timing is set at a start timing or an end timing of the chargestorage period.
 40. The method according to claim 37, wherein, in thecontrol step, a start timing or an end timing of the control pulse isdetermined in accordance with a duty ratio of the control pulse, that isdetermined by pulse-width modulation in accordance with the chargestorage period and a magnitude of an output signal from the imagesensing device, so as to generate the control pulse substantiallysymmetrically with respect to the reference timing along the time axis.41. The method according to claim 35, wherein the light source containsa plurality of color components, and in the control step, the lightsource to emit the plurality of color components is emitted inaccordance with the same control pulse.
 42. The method according toclaim 35, wherein, in the control step, a duty ratio of a control pulseis controlled by pulse-width modulation.
 43. A dimming control methodfor a light source in an image reading apparatus for irradiating anoriginal image with the light source and forming an image correspondingto the original image on an image sensing device through an imagingoptical system while scanning the original image in main andsub-scanning directions so as to read the original image, comprising: acontrol step of reducing barycenter movement of read positions of aplurality of color components in the sub-scanning direction, that isgenerated by afterglow characteristics of the plurality of colorcomponents contained in the light source for illuminating the originalimage.
 44. The method according to claim 43, wherein, in the controlstep, a phase of a control pulse for driving the light source iscontrolled so as to shift the ON start timing of the light source. 45.The method according to claim 44, wherein, in the control step, thecontrol pulse is generated symmetrically with respect to a predeterminedreference timing in the charge storage period along a time axis.
 46. Themethod according to claim 45, wherein, in the control step, thereference timing is set at a central position of the charge storageperiod along the time axis.
 47. The method according to claim 45,wherein, in the control step, the reference timing is set at a starttiming or an end timing of the charge storage period.
 48. The methodaccording to claim 45, wherein, in the control step, a start timing oran end timing of the control pulse is determined in accordance with aduty ratio of the control pulse, that is determined by pulse-widthmodulation in accordance with the charge storage period and a magnitudeof an output signal from the image sensing device, so as to generate thecontrol pulse substantially symmetrically with respect to the referencetiming along the time axis.
 49. The method according to claim 43,wherein the light source contains a plurality of color components, andin the control step, the light source to emit the plurality of colorcomponents is emitted in accordance with the same control pulse.
 50. Themethod according to claim 43, wherein, in the control step, a duty ratioof a control pulse is controlled by pulse-width modulation.
 51. A linesensor layout method in an image reading apparatus having a white lightsource containing R (red), G (green), and B (blue) color components withafterglow characteristics different from each other, and line sensors ofR, G, and B colors, which are laid out with an offset in a sub-scanningdirection, wherein a relative layout of the line sensors of R, G, and Bcolors is determined on the basis of the afterglow characteristics ofthe R, G, and B color components of the white light source.
 52. Themethod according to claim 51, wherein the line sensors corresponding totwo of the R, G, and B color components of the white light source, whichhave the largest difference in afterglow characteristics, are not laidout at two ends.
 53. The method according to claim 51, wherein the linesensors corresponding to the R and G color components are laid out attow ends, and a line sensor corresponding to the B color component islaid out at a center.